1. Field of the Invention
The present invention relates to a process for multifoaming expandable plastics, such as, for instance, EPS, EPE, EPP or polymer mixtures, with prefoaming being effected in a first step and afterfoaming of already prefoamed material being effected in at least one further step, as well as a device for afterfoaming expandable plastics, such as, for instance, EPS, EPE, EPP or polymer mixtures, prefoamed in at least one prefoaming stage.
2. Prior Art
In the context of foaming expandable plastics such as, for instance, EPS, EPE, EPP or polymer mixtures, it is known to foam such plastics to different desired bulk densities in continuous or discontinuous or batchwise foaming procedures in one stage or process step. When realizing a single foaming procedure or providing a single foaming stage, it is merely feasible to reach a limited bulk density. The expandable plastic is comprised of a thermoplastic and an embedded low-boiling hydrocarbon compound as a blowing agent, wherein heating, for instance by feeding water vapor for foaming, not only causes heating of the expandable plastic but also an increase in the volume and hence an accordingly low bulk density by the vapor pressure of the blowing agent rising.
In case lower bulk densities than those obtainable in a single foaming procedure are to be reached, it is, moreover, known to effect afterfoaming of an already prefoamed, optionally deposited material, in at least one further step, wherein continuous foaming procedures and foaming devices are known for such afterfoaming in order to attain the desired low bulk densities.
In connection with processes for foaming expandable plastics, there are, however, partially restrictive conditions imposed on account of increasingly tightened environmental regulations, which refer to the blowing agent content of expandable plastics or the releasable content of blowing agent contained in the offgas after having carried out an optionally multistage foaming procedure. In general, a relatively high portion or content of blowing agent is, however, required for the realization of continuously operating foaming procedures such that even with several foaming procedures optionally desired very low bulk densities cannot be attained, or only by employing complex and extensive additional means for purifying offgases drawn off the foaming procedures, within the context of a final continuous afterfoaming procedure.
The present invention aims at providing a process and a device for multifoaming or afterfoaming expandable plastics according to the initially defined kind, by which it is feasible to selectively attain very low bulk densities even by complying with strict environmental conditions.
To solve these objects, the process according to the invention for multifoaming expandable plastics is essentially characterized in that discontinuous or batchwise foaming is carried out as final afterfoaming. Due to the fact that, according to the invention, discontinuous or batchwise foaming is carried out as final afterfoaming, it is feasible to do with comparatively lower contents of blowing agent so as to readily comply with tight environmental conditions relating to the blowing agent portion contained in gases drawn off such a discontinuous or batchwise foaming procedure. By applying such discontinuous or batchwise afterfoaming, it is, moreover, feasible to attain also very low bulk densities, in particular considerably lower bulk densities than with continuous afterfoaming, wherein discontinuous or batchwise afterfoaming may be carried out also with precisely adjustable process parameters in respect of the bulk density to be attained and even for large volumes within a short time.
In order to attain or maintain precisely definable bulk densities upon completion of the afterfoaming procedure substantially independently of the bulk density of the at least once prefoamed material, it is proposed according to a further preferred embodiment of the process according to the invention that the prefoamed material is subjected to a weighing process prior to being introduced into final afterfoaming. By providing such a weighing process, it is feasible, in a manner precisely adapted to the bulk density to be attained and independent of the bulk density of the prefoamed material, to provide for said final discontinuous or batchwise afterfoaming an accordingly precisely defined amount of the at least once prefoamed material such that an accordingly precisely attained bulk density is ensured upon completion of the afterfoaming procedure.
In order to enable the rapid introduction of large filling amounts of already prefoamed material for carrying out the afterfoaming procedure, it is provided according to another preferred embodiment that the prefoamed material is introduced into a vessel for afterfoaming, using a closed-loop fluid conveying circuit. Such an introduction using a closed-loop fluid conveying circuit allows for short charging times of a vessel for discontinuous or batchwise afterfoaming, wherein blowing and sucking by means of a blower is effected simultaneously within said closed-loop fluid conveying circuit.
As already indicated above, it is necessary on account of increasingly tightened environmental regulations that the content of blowing agent contained in the offgases from the foaming stage be reduced to predetermined maximum limit values. In order to readily reduce the offgas to be given off by the foaming vessel, and the blowing agent contained therein, it is proposed according to a further preferred embodiment that offgas drawn off the final afterfoaming stage is at least partially recycled for conveying the prefoamed material into the afterfoaming stage. In that manner, a blowing agent content ranging within the permitted limit values can be selectively released to the environment and smaller dimensioned and hence simpler offgas purification means will do.
In order to obtain the desired end product, or the desired bulk density, it is, moreover, suggested that said final afterfoaming is carried out by introducing water vapor, optionally under the addition of air, at a pressure of 1 bar to 7 bars, in particular about 1.05 bar to 2 bars, at a temperature of from 75xc2x0 C. to 160xc2x0 C., in particular about 100xc2x0 C., as in correspondence with a further preferred embodiment of the process according to the invention. By observing the parameters indicated according to the invention in respect of the pressure and temperature to be applied in afterfoaming, the desired low bulk density may be guaranteed within a short period of time even for large batches, using, for instance, saturated vapor or slightly superheated vapor.
In order to obtain a homogenously expanded end product or a product having a homogenous bulk density, it is, moreover, suggested in a preferred manner that the material during afterfoaming is stirred under the continuous supply of water vapor.
To solve the above objects, it is, furthermore, provided according to the invention, for a device for afterfoaming expandable plastics, such as, for instance, EPS, EPE, EPP or polymer mixtures, prefoamed in at least one prefoaming stage, that it comprises at least one storage or supply means and a foaming vessel for a discontinuous or batchwise afterfoaming of the prefoamed expandable plastics. In this manner, the discontinuous or batchwise afterfoaming provided according to the invention may be carried out by means of a simple construction, wherein, in this respect, essentially a known foaming vessel for discontinuous or batchwise foaming including only slight modifications adapted to the purpose of use of the foaming vessel according to the invention for subsequent afterfoaming may be employed. In this context, it is, moreover, suggested that the foaming vessel, besides a feed opening for water vapor, a discharge duct for the foamed material and at least one offgas exhaust opening, comprises a tangentially connected blow-in opening for the prefoamed material and a sieve in the upper part of the foaming vessel as in correspondence with a further preferred embodiment of the device according to the invention. By providing a tangentially connected blow-in opening for the prefoamed material with an additional sieve in the upper part of the foaming vessel, rapid filling of the foaming vessel with the already prefoamed material already having a comparatively large volume is feasible in the desired quantity.
In order to enable precise proportioning of the prefoamed material aimed at attaining adjustable desired bulk densities, it is provided according to another preferred embodiment that a weighing means for the prefoamed material is integrated in the storage or supply means. Such a weighing means may be directly integrated in the storage or supply means, such weighing resulting in an accordingly good repeatability of the subsequent afterfoaming procedure and the thus attainable bulk density. In doing so, the weighing procedure as well as the following process conduct may be accordingly monitored and controlled by an electronic program scheduler.
In order to ensure the rapid and reliable introduction of the, in particular, previously weighed prefoamed material into the foaming vessel, it is provided according to a further preferred embodiment that a cellular wheel sluice or feeder is connected to the storage or supply means and that a blower is provided for introducing the prefoamed material into the foaming vessel. Such a cellular wheel sluice enables reliable proportioning from the storage or supply means after passage of the weighing means and the blower enables an accordingly rapid and safe introduction of the prefoamed material into the afterfoaming vessel.
In order to reach accordingly low blowing agent contents in the offgas, it is provided by the invention, according to another preferred embodiment, that a recycling duct for offgas drawn off the foaming vessel is connected to the foaming vessel for an at least partial recirculation of the offgas to the foaming vessel via the blower while forming a closed-loop conveying circuit for the introduction of the prefoamed material. By such an at least partial recirculation, it is feasible to not only do with accordingly small amounts of gas to be additionally fed for introducing the foamed material into the foaming vessel, but also avoid excessive loads on the environment. In this context, it is, moreover, provided by the invention, according to a further preferred embodiment, that an offgas purification plant is provided in an exhaust duct, whereby the arrangement of said recycling duct and said closed-loop fluid conveying circuit allows for accordingly smaller and simpler dimensioning of the offgas purification plant for reaching the specified offgas values.
Since the prefoamed material is subjected, in particular, to a weighing procedure prior to its introduction into the afterfoaming vessel in order to thereby enable precise apportioning of the prefoamed material aimed at attaining a desired bulk density, the foaming procedure or the attainment of the desired bulk density or volumes may be realized in the foaming vessel by simple means, to which end it is suggested that a filling level sensor or switch, which, in particular, is adjustable in height, is provided in the foaming vessel as in accordance with a further preferred embodiment of the device according to the invention. Such a filling level sensor or controller or switch, which, in particular, is adjustable in height, for instance, by adjustment enables simple adaptation to the bulk densities or volumes to be attained such that, for instance, for a wide range of bulk densities to be attained, the supplied and, in particular, weighed quantity of prefoamed material may be kept substantially equal and the precise adaptation to the desired bulk density is feasible within narrow tolerances merely by simply adjusting the filling level sensor or switch.
In order to ensure a uniform afterfoaming procedure, in particular when continuously blowing in water vapor during the foaming procedure, it is, moreover, preferably provided according to the invention that a stirring means is integrated in the foaming vessel, such a stirring means enabling the homogenous agitation of the afterfoaming material present in the foaming vessel by the water vapor introduced.